Science and technology

Why is the battery always running better at room temperature?

Improving the energy density and fast charge capability of lithium-ion batteries (LiBs) is the key to solving the cruising range problem and promoting the entry of electric vehicles (EVs) into the mainstream market. However, whether it increases the energy density or increases the fast charge rate, it will cause Dangerous lithium/lithium plating, which greatly degrades battery life. When the commercial graphite anode has a large polarization, the reaction potential will be lower than 0V vs Li/Li+. At this time, lithium deposition occurs, resulting in serious capacity loss and even a safety hazard.

It is generally believed that the phenomenon of lithium deposition is only severe at low temperatures and fast charge. However, recent studies have shown that lithium deposition is still a serious threat to the safety of high-energy batteries even at moderate charging rates and temperatures. To increase the energy density of the battery, it is necessary to increase the loading of the active material. The more the load increases, the more obvious the polarization, so that the phenomenon of lithium deposition is more likely to occur. Therefore, there is a ternary contradiction between fast charge, energy density and cycle life in commercial lithium batteries.

If the influence of temperature is taken into consideration, the ternary contradiction will be more complicated. In the early literature, it was generally believed that the battery was more likely to decay at high temperatures due to the faster growth of the solid-electrolyte-interface layer (SEI). When Waldmann tested the commercial 1.5Ah 18650 battery, it found that the battery had a longer cycle life at 25 °C, and above or below this temperature would cause the battery to decay faster.

Other studies have continued to discover this unusual phenomenon, but the reasons behind it are not known. Why is the battery always operating at room temperature better?

In response to the above problems, Prof. Chao-Yang Wang, Department of Mechanical Engineering, Materials Science and Engineering, Pennsylvania State University, USA, proposed a physical decay model combining lithium deposition and SEI growth, systematically studying fast charge, energy density and cycle life. The contradictory relationship between the factors. Using this model, it is possible to accurately detect the temperature at which the battery has the longest cycle life, and reveals the deeper reasons for increasing the charging rate and energy density to accelerate the degradation of battery performance caused by lithium deposition.